1. Field of the Invention
The present invention relates to a capacitor microphone unit and a capacitor microphone of which acoustic resistance can be readily controlled.
2. Description of the Related Art
Capacitor microphones include a capacitor microphone unit in which a diaphragm that vibrates upon receiving sound waves and a fixed electrode (also referred to as a “back electrode”) are placed opposite to each other with a spacer provided therebetween to form a capacitor. Capacitance of the capacitor changes when the diaphragm vibrates. Such a capacitor microphone unit is built inside a unit casing.
In such a capacitor microphone unit, acoustic resistance is provided to limit or control sound waves guided to the diaphragm to obtain a desired directionality. For example, acoustic resistance is provided by providing an acoustic resistance material that covers a sound communication hole on an insulating base as described in Japanese Utility Model Laid-open No. H07-29996.
The basic structure of a capacitor microphone unit that is incorporated in a capacitor microphone and includes an acoustic resistance material is as illustrated in FIGS. 6 and 7. In the capacitor microphone unit 1 as illustrated in FIG. 6, sound waves entering a rear acoustic terminal A pass through an air chamber provided with an acoustic resistance material 18 formed of a sponge or the like and a second acoustic resistance material 17 formed of a nylon mesh or the like to be applied to the rear side of the diaphragm. Thus, the capacitor microphone unit 1 has unidirectionality.
In FIG. 6, the capacitor microphone unit 1 is formed by installing the elements described below inside a unit casing 11 having a cylindrical shape. A side on which a bottom 11a is provided is the front side of the unit casing 11. The bottom 11a is provided with multiple holes 22 through which sound is guided, inside the capacitor microphone unit 1. A ring-shaped diaphragm holder 12 and a diaphragm 13 having the peripheral portion attached to one end surface of the diaphragm holder 12 are disposed at the position closest to the bottom 11a in the unit casing 11.
A fixed electrode 15 is arranged opposite to the diaphragm 13 with a ring-shaped spacer 14 provided therebetween. Naturally, a space defined by the thickness of the spacer 14 is provided between the fixed electrode 15 and the diaphragm 13. Thus, a capacitor is formed by the fixed electrode 15 and the diaphragm 13. The capacitance of the capacitor changes as the diaphragm 13 vibrates in accordance with the sound entering through the holes 22. The change in capacitance is output as a sound signal.
In the unit casing 11, an insulating base 16 that serves as a fixed electrode supporting member is disposed on the rear side of the fixed electrode 15. The insulating base 16 is formed of, for example, synthetic resin, and has a circular recessed portion on the front side (upper side as viewed in FIG. 6). The fixed electrode 15 is fitted in the recessed portion. A ring-shaped, further recessed portion is formed on an inner peripheral portion of the recessed portion. A ring-shaped, second sound resistance material 17 and an elastic acoustic resistance material 18 are fitted in the ring-shaped recessed portion. The insulating base 16 is provided with a hole extending in the thickness direction in the central portion. A terminal member 19 electrically connected to a field-effect transistor (FET) (not illustrated) that forms an impedance converter is fitted in the hole.
In the conventional capacitor microphone unit 1 as illustrated in FIGS. 6 and 7, an air layer that is the space between the fixed electrode 15 and the diaphragm 13 provides an acoustic resistance for controlling the vibration of the diaphragm 13. The acoustic resistance is adjusted by adjusting the number and the size of holes B penetrating through the fixed electrode 15 and the size of the space formed between the diaphragm 13 and the fixed electrode 15, for example.
However, in the case where the fixed electrode 15 is press molded for example, the size and the number of holes B cannot be readily changed.
The freedom of adjusting the acoustic resistance, by adjusting the space between the fixed electrode 15 and the diaphragm 13 through changing the thickness of the spacer 14, is limited because the spacer 14 is often formed by a commercially available plastic film and thus the choice of material is limited.